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HY-TRAK 100D 
SERVICE MANUAL 






J 
1 









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COPY NO. 



/S~^ 



DATE S./tQ.te 



APPROVED 




£)OMNITEST LIMITED 




The information contained herein is the 




property of and is supplied in confidence 




bv Omnitest Limited, and must not be 




disclosed or used either in whole or in 




part without the written- permission of 




the Company 





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INDEX 



1. SPECIFICATION AND WARRANTY 



2. USER GUIDE 



3. CIRCUIT DIAGRAMS 



4. TEST PROCEDURE 



5. ENGINEERING CHANGE NOTES RELEVANT TO HY-TRAK 100D 



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ISS.l 









1. SPECIFICATION & WARRANTY 

1 . 1 Specification 

1.2 Limited Warranty 



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SECTION 1/PAGE 1 



1 . SPECIFICATION & WARRANTY 



1 . 1 Specification 



Meter Section 

Ranges 

Accuracy 

Measurement Rate 

Display 

Measurement Technique 



200mA 2i% 2 On., 200A 

± 2% of reading ± 1 digit 

Approx. 3 per second 

3i digit LCD 

Dual Slope Integration 



Audio Section 

Ranges 

Hy-tone Resolution 

Audio Output 

Tone Delay 

(Open circuit to in-range) 



20mA 60mA 6 00mA 6A, 60n 

4mA 40mA 400mA 4X1 

250mW 

<50 milliseconds 



General 

Max. Voltage between Probe Tips 
Open Circuit Probe Voltage 
Probes 

Probe Isolation from Earth 
Overvoltage Protection 
Dimensions 

Weight 

Supply Voltage 
Supply Frequency 
Power Consumption 
Operating Temperature 



<20mV for Valid Reading 

Clamped to 600mV typical 

Removable, 4 wire Kelvin 
connected for accuracy 

>100KA 

to 240V ac 

75mm x 220mm x 240mm 
(excluding handle) 

1.2 Kg 

230V or 115V ± 10% 

50Hz - 50Hz 

12VA 

- 70°C 



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SECTION 1/PAGE 2 



1 .2 Limited Warranty 

For a period of 90 days from the date of its purchase new 
and undamaged from OMNITEST LIMITED, OMNITEST LIMITED or 
its authorised distributors will, without charge, repair 
or replace at its option, this product if found by it to be 
defective in materials or workmanship, and if returned to 
OMNITEST LIMITED or its authorised distributors transportation 
prepaid . 

This limited warranty is expressly conditioned upon the 
product having been used only in normal usage and service 
in accordance with instructions of OMNITEST LIMITED and not 
having been altered in any way or subject to misuse, negligence 
or damage, and not having been repaired or attempted to be 
repaired by any one other than OMNITEST LIMITED or its 
authorised distributors. With the exception of fuse replace- 
ment as specifically detailed herein. 

EXCEPT FOR THE FOREGOING EXPRESS WARRANTY OF REPAIR OR 
REPLACEMENT, OMNITEST LIMITED MAKES NO WARRANTY OF ANY KIND, 
INCLUDING BUT NOT LIMITED TO, ANY EXPRESS OR IMPLIED WARRANTY 
OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE, AND 
OMNITEST LIMITED SHALL NOT BE LIABLE FOR ANY DAMAGES, WHETHER 
DIRECT OR NOT OR OTHERWISE, BEYOND REPAIR OR REPLACING THIS 
PRODUCT. 



Omnitest Limited pursue a policy of continuous development 
and product improvement. The specification in this document 
may therefore be changed without prior notice. 






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.1 








■ 


2. 


USER 


GUIDE 



2 . 1 Overview 



2 . 2 Operations 



2 . 3 Maintenance 






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SECTION 2 /PAGE 1 



2. USER GUIDE 



2 . 1 Overview 



The HY-TRAK 100D is the first of a new generation of state of 
the art short circuit locators. Its high resolution digital 
meter and advanced circuit design ensure an instrument of un- 
paralleled performance whilst maintaining an ease of use 
suitable to operators at all levels. 

Although primarily designed to locate short circuits on 
printed circuit boards especially multi-layer, the HY-TRAK 
100D is eminently suitable for many other applications which 
require an accurate resistance measurement below the range of 
standard multimeters. Examples include checking the contact 
resistance of relays and switches, measuring the resistance 
of coils and motor windings (the HY-TRAK 100D uses a d.c. 
injection technique which ensures that there are no errors 
introduced by a.c. components) , checking for poor solder or 
wire-wrap joints, finding shorted output devices on bus- 
structured boards, etc. The instant tone feature of the 
HY-TRAK 100D ensures its suitability for continuity checking 
of cable harnesses and wire looms. 

The operator has the choice of using the meter or the audio 
tone to locate the short circuit. The lowest meter reading 
or the highest tone indicates the position of the short. 
'Dead' shorts (less than 2% of meter f.s.d.) are indicated by 
a doubling of the audio tone frequency. This Hy-tone feature 
is unique to the HY-TRAK 100D. 

The HY-TRAK button introduces a super-sensitive 20 milliohm 
full scale tone range to produce sensible tone gradients on 
low resistance power tracks whilst maintaining the meter on 
200 milliohm range to guarantee legible jitter- free readings. 
The d.c. injection (which prevents decoupling capacitors from 
introducing 'false' shorts as would be the case on an a.c. 
instrument) and the HY-TRAK range ensure that shorts between 
voltage planes or power tracks present no problems to the 
HY-TRAK 100D, which was specifically designed with this 
application in mind. 

As both probes are fully floating the HY-TRAK 100D can be used 
on earthed equipment without any fear of false readings due to 
earth loops. 









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SECTION 2/PAGE 2 



Warning 

Due to the very sensitive DVM input, the HY-TRAK 100D should 
only be used on items that are switched off and with all 
capacitors discharged . 

If the HY-TRAK is connected in error to a voltage source the 
internal protection circuitry will prevent permanent damage. 
However, it may be necessary to replace certain fuses - see 
section 'Fuse Replacement'. 



2.2 Operations 

(a) Handle Adjustment 

Grip the handle arms adjacent to the pivot points and 
press inwards. Maintaining the pressure, swivel the handle 
to the desired position and release. Handle will now lock 
in this position. 

(b) Initial Checks 

Turn the instrument on via the mains on/off switch incor- 
porated in the volume control. The red L.E.D. adjacent 
to the volume control should illuminate. A single digit 
1 should appear in LCD display if the probes are not 
touching . 

(If these indications do not appear, check mains supply 
and internal 100mA mains fuse.) 

Turn volume control fully clockwise. Touch probes together. 
A tone should be heard. Adjust volume control to comfor- 
table listening level. 

The meter should display a reading. (The reading will not 
necessarily be zero. See 'Calibration' section.) 

(c) Calibration 

It is only necessary to calibrate the instrument if precise 
resistance readings are required. Normally, when checking 
for short circuits, one is looking for the minimum reading, 
the actual value is immaterial. 

(i) Allow instrument to warm up for 15 minutes with 
probes not touching. 

(ii) Set to 200 ohm range. 

(iii) Short probe tips together. 



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(iv) Adjust 'zero' potentiometer with small screwdriver 
to obtain a reading of 00.0 on the LCD display. 
There should not be a - sign. 

(v) Switch to 20 ohm range. Check reading is 00.0. 

(vi) Reading may be typically 1-2 milliohms on 2 ohm 

and 200 milliohm ranges due to resistance of probe 
tips from Kelvin junction. 

Insert probes into calibration resistor sockets on 
rear panel. (Ensure instrument is flat on the 
bench. ) 

(vii) Set to 200 ohm range. 

(viii) Adjust 'cal* potentiometer to obtain reading of 
100.0. 

(ix) Remove probes from calibrate resistor sockets. 

Short probes together. Check reading is still 00.0. 

Your HY-TRAK 100D is now calibrated to within the accuracy 
specification . 

(d) To Locate a Short Circuit on a P.C.B . 
(i) Set to 200 ohm range. 

(ii) Connect probes across suspect tracks. 

(iii) If tracks are shorted the LCD will display a reading. 

Holding one probe steady, move the other along the track 
until you obtain the lowest meter reading and highest tone. 
You are now very close to the short. Select higher 
sensitivity ranges, if required, to precisely locate the 
short. 

Use the HY-TRAK range on power tracks and earth planes to 
obtain a greater sensitivity on the audio range. 

(e) Resistance Measurements 

Ensure instrument is calibrated. Select appropriate range 
and read ohmic value from LCD display. If measuring 
suspect solder joints, contact resistance of relays, etc. 
do not apply excessive pressure to the probes or you may 
affect the resistance of the item under test. 



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ISS1 SECTION 2 /PAGE 4 



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2.3 Maintenance 

This will only be necessary if the instrument has been connected 
to a 'live' circuit. (See 'Warning') 

(a) Turn instrument off. 

(b) Disconnect from a.c. mains supply. 

(c) Remove 4 'posidriv' screws from base of instrument. Lift 
lid of case. 

(d) Remove fuses one at a time and check continuity. 
Only replace fuses with correct type and value . 
Using an incorrect type or value of fuse will either: 

(i) Affect the accuracy of the instrument, or 

(ii) Render the overvoltage protection inoperative, or 
(iii) Both (i) and (ii) above. 

We recommend the Schurter FSF and FST type available from 
Messrs Verospeed Ltd., Stansted Road, Boyatt Wood, Eastleigh, 
Hants. S05 4ZY. 

Part No. 228-23437A 100mA FST 
228-23474E 200mA FSF 
228-23477G 400mA FSF 

Or direct from Omnitest Limited. 

The correct value of each fuse is: 

Fl 100mA Antisurge Mains 

F2 400mA Quick Blow D.C. Inject 

F3 200mA Quick Blow Measurement I/P 

F4 400mA Quick Blow D.C. Inject 

F5 200mA Quick Blow Measurement I/P 

(e) Replace lid and tighten 4 retaining screws. 

(f) Reconnect to a.c. mains, switch on and verify correct 
operation of instrument. 



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3. CIRCUIT DIAGRAMS 






SHT. 1. Voltage Regulator /Constant Current 

SHT. 2. Current Driver Stage 

SHT. 3. Differential Input Amplifier 

SHT. 4. Audio Section 

SHT. 5. Decimal Point Control & L.C.D. 

SHT. 6. Power Supply 




NOTE: 






(a) Sheets 1. through 6. above represent the current 
manufactured status of the HY-TRAK 100D. 

(b) Sheet 3A represents the details of sheet 3. before 
the implementation of ECN 101. 

(c) Circuit diagram balloon notation used 

















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SW2 SW3 SW4 SW5 SW6 



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TITLE. VOLTAGE REGULATOR/ CONSTANT CURRENT. 





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VR1 (Not used by customer) 



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TITLE. DIFFERENTIAL INPUT AMPLIFIER 



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4. TEST PROCEDURE 



4 . 1 Test Equipment 



4.2 Test & Set Up Procedure 












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SECTION 4/PAGE 1 



4. TEST PROCEDURE 



4. 1 Test Equipment 

(a) DVM (Typical instrument Fluke 8024B) . 

(b) Bench D.C. Variable P.S.U. 0-30v. 

(c) Decade Resistance Box. 

(d) Test lamps (4 off) 




ITEM 

1 
2 



PART (RADIO SPARES) 



RS 587-074 
RS 423-790 






4.2 Test & Set Up Procedure 
General 






This procedure covers the Test & Setting Up operation for both 
issues of the HY-TRAK 100D to date. The essential differences 
between the two issues are covered in the associated Engineering 
Change Note ECN 101. 

The full procedure comprises of 10 stages. It is important to 
note that with regard to the two issues of instrument, steps (e) 
through (j) are identified as (E) through (J) respectively in 
relation to those units produced prior to the issue of ECN 101. 

(a) Insulation Tests (Do not connect unit to mains.) 

(i) With DVM on 32 MA range: 

Check for coOhms between E and N, and E and L mains 
terminals. 



(ii) With DVM on 320A range: 






1. Check between N and L for approx. 190D. (240V 
tapping) or approx. 40O-(110V setting). 

2. Check for zero Ohms from mains E terminal to 
0V link on centre of PCB. 



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SECTION 4 /PAGE 2 



(iii) Visually check mains tappings and orientation of 
transformer. The five plastic lugs on the top 
should be facing the rear of the PCB. 

(iv) Visually check that capacitor C3 has been removed 
from the Anders DPM module. 

(v) (ONLY FOR UNITS NOT INCORPORATING ECN 101) 

1. Ensure IC5 position has an IC base fitted. 

2. Ensure that IC's (741) are fitted to position 
IC3/IC4 and IC5. Experience has shown Motorola 
devices to give the best operation. 

(b) Protection Tests (Ensure that the unit is disconnected 

from the mains . ) 

(i) Remove fuses FS2-FS5. Attach the test lamps across 
each fuse position (4 off) . 

(ii) Connect probe leads. 

(iii) Set PSU (Bench supply) to 10V +.1V. 

(iv) Connect PSU -ve to HY-TRAK earth. 

(v) Touch one probe tip to PSU +ve, check that two 
lights are lit. 

(vi) Touch other probe tip to PSU +ve, check that the 
other two lights are lit. 

(vii) Connect PSU +ve to HY-TRAK earth and repeat steps 
(v) and (vi) using PSU -ve. 

(viii) Disconnect PSU. 

(ix) Replace fuses as follows: 

1. FS2 and FS4 with 400mA. 

2. FS3 and FS5 with 200mA. 



(c) D.C. Voltage Checks 









(i) Connect HY-TRAK to mains and turn on: LED should 
light. 



(ii) 



With DVM on DC Volts 32 Volt range, check: 

1. Unregulated rails at ±22V ±2V on bridge rectifier 

2. ±15V supplies on links at centre of PCB (±0.5V) 



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SECTION 4 /PAGE 3 



: 



3. +4.7V supplies on IC 10 pin 7 & 14 (±1.0V) 
(±0.5V)** 

4. +0.7V supply on IC 9 pin 16 (±0.2V) (±0.1V)** 
** Use these values on units not modified by ECN 101. 

(d) D.C. Current Inject Checks 

(i) With DVM on DC Volts 3200mV range, connect between 
ground and TR3 emitter. (This is L.H. end of large 
lQrt resistor at rear L.H. of switch bank.) 

Ensure that this voltage can be set to IV (±10mV) 
using VR2 (Centre pot at rear of PCB) . 



(ii) 






With DVM on DC mA 800mA range connect HY-TRAK probes 
to DVM probes. You should get: 



HY-TRAK 


RANGE 


READING 


ACCURACY 


200/1 


100UA 


±1% 


20A 




1mA 


±1% 


2/v 




10mA 


±1% 


200m.fl. 




100mA 


±1% 


HY-TRAK 




100mA 


±1% 






(e) CMRR Adjustment 

Not applicable after ECN 101. 

(f) Funct ional Checks 
(i) 






(ii) 
(iii) 

(iv) 
(v) 



Short leads together on 20QA. range. Adjust VR3 to 
obtain 00.0 on LCD. 

Connect test leads to 100a cal. resistor in rear 
panel of HY-TRAK. Adjust VR2 to read 100. on 200.O. 
range . 

Connect HY-TRAK leads to resistance decade box. Set 
to lOOn.. Ensure HY-TRAK reads 100.0 ±1.1. 

Set Decade box to 0n. 

Set HY-TRAK to 'HY-TRAK' range. Ensure reading is 
approx . 70 . Om/1 ±2 0mA. 

Set to 200mO. range. Ensure reading has not changed. 

Set to 2n. range. Ensure reading is .070, i.e. 
correlation with reading on 200mn. range ±2 LSD's. 



(vi) 



Set to 20X1 range , reading should be 0.1. 



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i 



On all the above ranges check correct position of decimal 
point. 



200mn. 
2a. 
20/v 
200/x 



XXX. X 

x.xxx 

XX. XX 
XXX. X 



Some leading zeros will be blanked. 

(vii) Set Decade box to In. Readings should be: 



HY-TRAK 
200mTL 
2n. 
20/1 
200TL 



Overrange ( 1 ) 
Overrange 
l.xyz 
l.xy ±2 LSD'S 

1.x ±2 LSD's 



(viii) Set to Ida on Decade box. Reading should be: 



HY-TRAK 
200mn 
2/v 
2 On. 
200-n. 



Overrange 
Overrange 
Overrange 
10. xy 
10.x 



±2 LSD's 



NOTE: Some warm up drift may occur over the first 15 minutes 
after switch on. 

(g) IC5, Select on Test 

On 200n range short probes together. Set volume to zero. 

Note reading. Turn volume to full. Reading should not 

vary by more than three least significant digits, preferably two. 

(h) Setting Up Audio Section 

Connect HY-TRAK to 1 On. pot. 
Set to 20n. range. 

(i) Set pot until display reads 6.0. 

Adjust VR4 until tone just starting to come in. 
Set to 6 . in. - ensure no tone . 
Seal VR4 with nail varnish. 

(ii) Set pot to 4n, set HY-TRAK to 200n. range. 
Adjust VR5 until you just get HY-TONE. 
Set pot to 4.2n - ensure no HY-TONE. 
Seal VR5 with nail varnish. 

(j) R14 S.O.T . 

Stay on 200n. range, ensure R14 is 4.7Kn. 



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Touch one probe to 0V - you should not get a tone. 

Touch other probe to 0V (after removing the first) - 
ensure no tone. 

If necessary reduce value of R14 until tone disappears for 
both the above tests. 

(k) Final Checkout 

Use the instrument on a PCB. Ensure that HY-TRAK audio 
starts at about 20mn. 

Ensure that 200ma tone starts at 60mn. 

Ensure that HY-TONE starts at 2% of FSD, i.e. 4m/ton 200mn. 
range. 

Ensure that you get progressive tone slide and that tone 
quality is good. 

Check ranges all function. 

































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SECTION 4 /PAGE 6 



(E) CMRR Adjustment (BEFORE ECN 101 ONLY) 

Disconnect standard HY-TRAK leads. Connect CMRR leads. 
Connect to PSU (multiturn side) observing conventional 
colour code of polarity. Select PSU 0-1 5V range and set 
to 0V ±20mV. 

(i) Set HY-TRAK DPM reading to 00.0 (20CUX range) using 
VR3 (zero) pot. Set PSU to 1.000V ±50mV. 

Set HY-TRAK DPM to 00.0 using VRl (CMRR) pot. 

Reset PSU to 0V ±20mV, check HY-TRAK DPM still 00.0. 

If not goto (i) and keep repeating procedure until 
DPM reads at both 0V and IV. 

Seal VRl (CMRR) pot using nail varnish. Remove CMRR leads. 

Reconnect standard test leads. 

(F) Function Checks (BEFORE ECN 101 ONLY) 

(i) Connect standard test leads to 100a cal. resistor 
in rear panel of HY-TRAK. Adjust HY-TRAK DPM to 
read 100.0 on 20On. range. 

(ii) Connect HY-TRAK leads to resistance decade box. 
Set to 100n. Ensure HY-TRAK reads 100.0 ±1.1. 

■ 

(iii) Set Decade box to 0n.. 

Set HY-TRAK to 'HY-TRAK' range. Ensure reading is 
approx. 70.0mil±10ma. 

(iv) Set to 200mA. range. Ensure reading has not changed. 

(v) Set to 2A range. Ensure reading is .070, i.e. 

correlation with reading on 200mA range ±2 LSD's. 

(vi) Set to 20n range. Reading should be approx. 0.07. 

(vii) Set to 200a range, reading should be 0.1. 

On all the above ranges check correct position of decimal 
point. 



200mA 
2a 
20/v 

200n. 



xxx. x 
x.xxx 
xx. xx 
xxx.x 



Some leading zeros will be blanked. 



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(viii) Set Decade box to La. Readings should be: 



HY-TRAK 


Over range 


(1 ) 


200mn 


Overrange 




2a. 


1 . xyz 




20a 


l.xy 


±2 LSD'S 


200rv 


1.x 


±2 LSD's 



(ix) Set to 100- on Decade box. Readings should be: 



HY-TRAK 


Overrange 




200mA 


Overrange 




2a 


Overrange 




20tv 


10. xy 




200a 


10. x 


±2 LSD's 



NOTE: Some warm up drift may occur over the first 15 minutes 
after switch on. 

(G) IC3, IC4, IC5, Select on Test (BEFORE ECN 101 ONLY) 

On any HY-TRAK range short probes together. Set volume to 
zero. Note reading. Turn volume to full. Reading should 
not vary by more than three least significant digits, 
preferably two. 

If it does, try different IC3's, IC4's and IC5*s until you 
find a suitable set. 

This problem is solved by the implementation of ECN 101. 

NOTE: In theory, changing IC3, 4 and 5 should not change 
the CMRR setting. In practice, it is worth going 
over section (E) (i) again. 

(H) Setting Up Audio Section (BEFORE ECN 101 ONLY) 

Reconnect HY-TRAK to resistance decade box. 
Set to 200a range . 

(i) Set decade box to 60ji. 

Adjust VR4 until tone just starting to come in. 
Set to 6 La - ensure no tone. 
Seal VR 4 with nail varnish. 

(ii) Set decade box to 4A. 

Adjust VR5 until you just get HY-TONE. 
Set to 5/1- ensure no HY-TONE . 
Seal VR5 with nail varnish. 



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SECTION 4/PAGE 8 



: 



(J) R14 S.O.T . (BEFORE ECN 101 ONLY) 

Stay on 200A range . 

Connect decade box via croc clip leads, to R14 terminals - 

Set to approx. 10KA. 

Touch one probe to 0V - you should not get a tone. If you 

do, reduce decade box until tone just disappears. 

Touch other probe to 0V (after removing the first) . 

Ensure no tone. Again reduce decade box if necessary until 

tone just disappears. 

Note decade box reading. 

Connect nearest standard value resistor across R14, (probably 

best with E-Z clips first) . 

Again touch probes to 0V and ensure no tone. (If tone, 

reduce resistor value 1 value) . 

Solder R14 into place. 



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5. ENGINEERING CHANGE NOTES RELEVANT TO HY-TRAK 100D 

5.1 101 - Solves Drift and CMRR Adjustments 

5.2 102 - Earth Bonding 

5.3 103 - Modified DPM Unit 















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SECTION 5 /PAGE 1 



5 . ENGINEERING CHANGE NOTES RELEVANT TO 100D 



5.1 ECN 101 - Drift & CMRR Adjustments 
Problem : 
Elongated setting up time due to: 

(a) Selecting IC's 3, 4, & 5 for zero drift with volume. 

(b) Setting up CMRR pot. 

Solution for 1 : 

(a) Remove IC's 3 and 4 and associated IC sockets. 

(b) Replace resistors R21 and R22 with links. 

(c) Link IC3.3 to IC3.2. 

(d) Link IC4.3 to IC4.2. 

(e) Wire IC4.1 to IC5.1 (also see (g) below). 

(f) Wire IC4.5 to IC5.5 (also see (g) below) . 

(g) Insert Tant. capacitor 2.2uf 16v between IC4.1 and IC4.5 
(polarity immaterial) . 

(h) Replace link idented -15V (near centre of PCB) with 470/1 

l/4w 5% resistor. 
(j) Replace C5 with lOuf 16V electrolytic capacitor (polarity 

+ to 0V connection) . 
(k) Cut track going to IC5 pin 4. 
(1) Wire from new C5 cap -ve to IC5.4. 
(m) Remove existing loudspeaker earth wire. 

Reroute from loudspeaker to 0V link (near centre of PCB) 

or as close as possible to transformer 0V. 



Solution for 2 : 

(a) Replace R26 (9K La) with lOKn. 1% lOOppm resistor. 

(b) Replace VRl with link. 

No CMRR adjustment is now required. 






5.2 ECN 102 - Earth Bonding 

SAFETY WARNING TO ALL OMNITEST HY-TRAK 100D USERS 

THE HY-TRAK 100D SHOULD NOT BE USED UNTIL THE FOLLOWING 
MODIFICATIONS ARE CARRIED OUT. 



Problem : 

The front and rear panels of the HY-TRAK 100D are metal and are 
not earthed. As such this constitutes a potential health hazard 
and Omnitest Limited recommends that the following modifications 
are made to each instrument immediately. 



RESTRICTED AND COMMERCIAL IN CONFIDENCE 



ISS.l 



SECTION 5 /PAGE 2 



Solution: 



(a) 
(b) 
(c) 



(d) 
(e) 



(f) 



(g) 

(h) 

(J) 
(k) 

(1) 



(m) 



(n) 



(P) 



(q) 



(r) 



Disconnect the instrument from the mains supply. 

Remove the probes from the front panel. 

Remove 4 countersunk screws from the base of the instrument. 

Lift off the lid and sides of the case complete with the 

handle . 

Remove knob from on/off volume pot. (This simply pulls off.) 

Unscrew the nut now revealed and slide on/off volume pot to 

the rear. It is not necessary to unsolder the wires to this 

pot. 

Add one 3/8" diameter solder tag to pot shaft, reposition in 

front panel and retighten retaining nut. We recommend the 

application of zinc paste to the front panel to prevent future 

oxidisation of the connection. 

Unsolder the Earth, Live and Neutral wires from the rear of 

the printed circuit board, adjacent to the mains transformer. 

Remove the rear panel. Drill and countersink a 4ba clearance 

hole in a suitable position. 

Solder a 4ba solder tag to themains earth wire (yellow/green) . 

Solder a yellow/ green wire in 16/0.2 gauge of suitable length 

to the earth point on the PCB. Terminate in a 4ba solder tag. 

Solder a yellow/green wire in 16/0.2 gauge of suitable length 

to the 3/8" solder tag on the on/off volume pot. Terminate 

in a 4ba solder tag. 

Bolt these three solder tags to the rear panel using a 4ba x i" 

countersunk screw, shakeproof washer and nut. Again the use 

of zinc paste on the rear panel is recommended. 

Resolder the Live (brown) and Neutral (blue) wires to the L 

and N terminals on the PCB. The connections should be sleeved 

with silicon rubber or heat-shrink sleeving. 

Replace the loudspeaker retaining screw situated near the 

centre of the PCB, with a M3 x 20mm countersunk nylon screw. 

Do not overtighten. 

Replace lid being careful to slide the front and rear panels 

into the appropriate grooves. Replace the 4 countersunk 

screws in the base. 

Replace the probes, reconnect the instrument to the mains and 

check the instrument still functions correctly. 



This modification scheme should not affect the calibration of the 
instrument at all. 



NOTICE 

If you do not wish to carry out this modification yourselves, 
.the instrument should be returned, suitably packed and post paid, 
to Omnitest Limited, where we shall be pleased to modify your 
instrument free of charge. There will, however, be a small fee 
to cover the return post and packing of the instrument. 



The instrument should be clearly labelled 'Returned for earth 
modification' . 



RESTRICTED AND COMMERCIAL IN CONFIDENCE 
ISS.l SECTION 5/PAGE 3 



5.3 ECN 103 - Modified PPM Units 

This ECN only applies to any unit that had to use an Anders OEM2 
to replace an OEM1 DPM. 

Problem : 

The OEM2 when used as a direct replacement for the OEMl causes 
a high current drain on the +4.7V supply. 

Dl, D2 and D3 inputs control the decimal point position on the 
LCD. These are actually inputs to a CMOS 4070 chip on the 
Anders OEM2. This chip has VDD at +4.7V (i.e. direct from our 
HY-TRAK +4.7V supply) but we now discover that VSS is not 
connected to -4.7V but to its own internal logic zero rail 
(about -2V) . Thus when we drive Dl , D2 or D3 to our logic zero 
of -4.7V, we are applying a negative voltage below the VSS of 
the chip which is turning on the input protection diodes and 
causing the high current drain on the VDD supply. 

Solution : 

We must connect pin 7 of IC10 and the -ve rail to switches 3 and 
4 of the HY-TRAK to the logic zero rail of the OEM2. 

This involves 2 track cuts and two links to be fitted. 

The logic zero rail of the OEM's is available at the 'TEST' 
connection of the 0EM2. This is linked to the TST pin of the 
HY-TRAK. 

Rl and R2 on the HY-TRAK should now be lKn. - this gives us a 
potential 10mA supply which is ample for the 0EM2 and IC10 under 
normal conditions. 

Mod . Scheme : 

1. Cut track at IC10 pin 7. 

2. Cut -4.7V rail between switch 3 and VS pin. 

3. Link TST pin to IC10 pin 7 and switch 3 formerly -4.7V rail 
pin. 

Application : 

(a) This mod scheme should be applied to all HY-TRAKs fitted with 
OEM2's, i.e. Serial No. 1110 onwards. 

(b) Any older unit that has to be repaired with a new DPM.